Security inspection is of great importance in fields such as anti-terrorism and fighting against trafficking in drugs and smuggling. After 9/11 terrorist attacks of the United States, countries in the whole world more and more stress security inspection, and particularly take a series of security inspection measures to rigidly inspect passengers' luggage and articles and cargo containers at public sites such as airports, bus stations, the Customs House and docks.
At present, the mainstream imaging technology extensively used for security inspection systems is radiation imaging technology. According to exponential decay theory of radiation rays (photons), the radiation imaging technology works as follows: a radiation source is used to irradiate an inspected object from one side of said inspected object; the rays, after transmitting through the inspected object, are received by a ray collecting means; the ray collecting means converts the received rays into transmission data in digital form; the transmission data are combined into projection data which are then output to a computer for imaging; and the computer processes the collected data, synthesizes or reconstructs an image and displays it. A security inspection system using radiation imaging technology can carry out tomographic imaging or radiographic imaging. Tomographic imaging shows a tomographic image of the inspected object and combines multiple layers of tomographic images into a 3-dimensional image; radiographic imaging shows a 2-dimensional perspective image of the inspected image.
Since tomographic imaging requires the ray collecting means to receive all-round irradiation to the inspected object to obtain transmission projection data of the ray beams, a tomographic imaging security inspection system generally needs a computed tomography (CT) apparatus wherein at least one of the inspected object and the radiation source needs to rotate. In practical application, a security inspection system generally is required to inspect in an on-line real-time manner with a fast imaging speed. For example, articles carried by civil aviation are inspected at the Customs House at a rate of 0.5 m per second, so it is hard for even a spiral CT apparatus with a large screw pitch to meet the foregoing requirement. Besides, for large-size objects such as containers at the Customs House, it is very difficult for a container or a radiation source to rotate. Additionally, a CT apparatus is costly. Because of the above factors, security inspection systems using a CT apparatus for 3-dimensional imaging are not extensively applied.
In contrast with the tomographic imaging security inspection systems, a radiographic imaging security inspection system is widely used at public sites such as airports, bus stations, the Customs House and docks. However, the radiographic imaging security inspection system cannot avoid the overlapping effect of the objects in the direction of rays, and cannot solve the overlapping deficiency of the objects in the direction of rays so that the inspection capacity of the radiographic imaging security inspection system is seriously insufficient.